Numerous efforts have been made to develop aids for the blind based on conversion of optical images to auditory or tactile stimuli. However, the limited performance of such conversion devices, and the difficulty in training patients to interpret the converted signals, have seriously hampered practical application.
In the early 1930's, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart.
Following the advent of intraocular surgical techniques (i.e., pars plana vitrectomy), Dawson and Radtke stimulated cat's retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that (1) electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses; (2) there was little increase in cortical response when stimulus currents were above 1.5 mA; and (3) there was a decline in cortical response as the stimulus current was increased. These experiments were carried out with needle-shaped electrodes which penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson).
The possibility of implanting a more permanent intraocular prosthetic device to electrically stimulate the retina became feasible only recently after the introduction of retinal tacks in retinal surgery. De Juan et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas which had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan et al., 99 Am. J. Ophthalmol. 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina up against the choroid and the posterior aspects of the globe.
While a means of attaching a retinal implant now appears in hand, there has not yet been provided an electrode suitable for chronic implant in association with the retina. Needle electrodes are difficult to fabricate individually--the possibility of fabricating a uniform bed of such electrodes would appear significantly more difficult. Moreover, penetration of the retinal surface, or basement membrane, can impart mechanical damage to the cells of the retina and the ganglion cell axons at the surface thereof. Finally, the placement of an electrode tip in close proximity to a neuron, on a chronic basis, raises the possibility of chemical damage to the neuron as a result of ion flux. Applicants are aware of nothing in the literature which obviates these problems.